ERT 319ERT 319Industrial Waste Industrial Waste

Treatment Treatment

Semester 1 2013/2014Semester 1 2013/2014

Solids Waste Treatment &

Disposal

Introduction• Constituents removed in WW treatment

plant:– Screenings, Grit, Scum, Solids and biosolids (also

known as sludge)– Biosolid - Class A & Class B

• Handling of solids and biosolids are complex because:1. It composed largely of substances responsible for the

offensive character of untreated WW2. Portion of biosolids produced from biological treatment is

offensive3. Only a small part is solid matter

SOLID PROCESSING METHODS (page 1450)

Solids Sources, Characteristics, & Quantities

Sources - Vary according to the type of plant and its method of operation

Characteristics- Vary depending on the origin of the solids, the amount of aging that

has taken place, and the type of processing to which they have been subjected.

GENERAL COMPOSITION

Quantities

- The volume of sludge depends mainly on its water content and only slightly on the character of the solid matter.

- A 10 % sludge, for example, contains 90 % water by weight. - If the solid matter is composed of fixed (mineral) solids and volatile

(organic) solids, the specific gravity of all of the solid matter is:

- Volume of sludge:

Ssl = specific gravity of sludge

Also, the volume varies inverse with percent of solid matter in the sludge,

matter solidof percentP

volume sludgeV

P

P

V

V

2

1

2

1

Example 14-1 Volume of Untreated and Digested SludgeDetermine the liquid volume before and after digestion and the

percent reduction for 500 kg (dry basis) of primary sludge with the following characteristics:

Primary Digested

Solids, %

Volatile matter, %

Specific gravity of fixed solids

Specific gravity of volatile solids

5

60

2.5

≈ 1.0

10

60 (destroyed)

2.5

≈ 1.0

Solution (pg 1459)

1. Compute the average specific gravity of all solid2. Compute the specific gravity of primary sludge3. Compute the volume of primary sludge4. Compute the percentage of volatile matter after digestion5. Compute the average specific gravity of all solid in digested

sludge6. Compute the specific gravity of digested sludge7. Compute the volume of digested sludge8. Determine the percentage reduction after digestion

PFRP and PSRP –to meet pathogen and vector reduction requirementPSRP –reduce but do not eliminate!

Solids Processing Flow Diagrams

THICKENING

THICKENING• Used to increase solid content of sludge by removing

removing a portion of liquid fraction• Usually accomplished by physical means; e.g settling,

flotation, centrifugation, gravity belt and rotary drum.• Why is it important?

1. Beneficial to the subsequent treatment processes from the following stand points:• Capasity of tanks and equipment required• Quantity of chemical required for sludge conditioning• Amount of heat (in digester) and fuel required for heat drying

and incineration

2. Cost reduction-small pipe size and pumping cost

3. Liquid sludge can be transported easily

• In designing thickening facility, it is important to:– Provide adequate capasity to meet peak demand– Prevent septicity, with its attendant odor

problem.

Gravity belt thickener

Schematic diagram : CENTRIFUGAL THICKENING

STABILIZATION--DIGESTIONS--

• Stabilization process of sludges for volume reduction, production of usable gas (methane), and improving the dewaterability of sludge

• Solids and biosolids (sludge produced from primary or secondary treatment) are stabilized to:- reduce pathogens- eliminate offensive odors- inhibit, reduce, or eliminate the potential for

putrefaction (decay, decompose of organic matters). Therefore, stabilization involves the reduction of volatile

content and addition of chemicals to solid and biosolid

Not suitable for survival of microb

Alkaline Stabilization- A method to eliminate nuisance conditions in sludge is

through the use of alkaline material to render the sludge unsuitable for the survival of microorganisms.

- In alkaline stabilization process, lime is added to untreated sludge in sufficient quantity to raise the pH to 12 or higher.

- When the pH is high: creates environment that stops or substantially retards the microbial reactions that can otherwise lead to odor production and vector attraction.

the sludge will not putrefy, create odors, or pose a health hazard so long as the pH is maintained at this level.

also inactivate virus, bacteria and other microbes present.

Application of Alkaline Stabilization Processes:1)Lime Pre-treatment

- addition of lime to sludge prior to dewatering- addition of lime to sludge prior to dewatering

2) Lime Post-treatment- addition of lime to sludge after dewatering

3) Advanced Alkaline Stabilization

ANAEROBIC DIGESTION

• Involves the decomposition of organic and inorganic matter (principally sulfate) in the absence of molecular oxygen.

• Major application: in the stabilization of concentrated sludge produced from the treatment of municipal and industrial wastewater.

• Can produce sufficient digester gas (methane) to fulfill the energy (heat & electricity) needs.

Design Factors of Anaerobic Digester.1) Solids & Hydraulic Retention Times

- Solids retention time (SRT) average time the solids are held in the digestion process (Mass / day)- Hydraulic retention time, τ average time the liquid is

held in the digestion process (Volume /day)- Digestion systems without recycle SRT = τ- Hydrolysis, fermentation and methanogenesis (3- reactions in anaerobic process) are directly related to SRT- If the SRT < min. SRT in each reaction, bacteria cannot grow

rapidly enough digestion fails

2) Temperature- determines rate of digestion - rates of hydrolysis &

methane formation- mostly operates in mesophilic temperature - between 30-38⁰C (or thermophilic T 50-57 ⁰C)- important since bacteria of methane-formers are

sensitive to temperature changes (ex: ΔT > 1 ⁰C/d affect performance, thus ΔT < 0.5 ⁰C/d is recommended)

3) Alkalinity- Ca, Mg and ammonium bicarbonates are examples of buffering substances found in a digester.- Digestion process produces ammonium bicarbonate from the breakdown of protein in the raw sludge feed (others found in feed sludge)- The concentration of alkalinity proportional to the solids feed concentration.- Alkalinity mostly in well-established digesters 2000 – 5000 mg/L.

- The principal consumer of alkalinity is CO2, i.e., CO2 is produced in fermentation and methanogenesis phases . Due to partial pressure of gas in a digester, the CO2 solubilizes and forms carbonic acid, which consumes alkalinity.- Therefore, CO2 concentration is reflective of the alkalinity requirements.

Single-stage High Rate DigestionHeating, auxiliary mixing, uniform feeding, and thickening of

feed stream- The sludge is mixed by gas recirculation, pumping, or draft-

tube mixers (separation of scum & supernatant does not take place), and the sludge is heated to achieve optimum digestion rates.

- With fixed roofs or floating covers

Two-stage Digestion- a high-rate digester is coupled in series with a second tank. - 1st tank: used for digestion, heated & equipped with mixing

facilities- 2nd tank: unheated, used for storage- This type of digester is seldom used because of expense of

large tank building and negligible benefit of 2nd tank.

Anaerobic digester

Example 14-5 Estimating Single-stage, High Rate Digester Volume and Performance

Estimate the size of digester required to treat the sludge from a primary treatment plant designed to treat 38 000 m3/d of wastewater. Check the volumetric loading, and estimate the amount of gas produced per capita. For the wastewater to be treated, it has been found that the quantity of dry volatile solids and biodegradable COD removed is 0.15 kg/m3 and 0.14 kg/m3, respectively. Assume that the sludge contains about 95% moisture and has a specific gravity of 1.02.

Other pertinent design assumptions are as follows:1. The hydraulic regime of the reactor is complete-mix2. SRT = τ = 10 days at 35 ⁰C3. Efficiency of waste utilization (solids conversion) E = 0.704. The sludge contains adequate nitrogen and phosphorus for

biological growth.6. Constants are for a temperature of 35 ⁰C. 7. Digester gas is 65 % methane. 8. Volume of methane produced: 1359 m3/d

Aerobic Digestion• To meet Processes to Significantly Reduce Pathogens

(PSRP) requirement (see Table 14-9, 14-10, 14-11 & 14-12, pg 1462-1465)

• Used to treat: waste-activated sludge only mixtures of waste-activated sludge or trickling filter sludge

and primary sludge Or waste sludge from extended aeration plants

• Plant capacity may up to 2 m3/s

Advantages compared to anaerobic digestion:

i- volatile solids reduction in a well-operated aerobic digester is approximately equal to that obtained anaerobically

ii- lower BOD concentrations in supernatant liquoriii- production of an odorless, humuslike, biologically stable end

productiv- recovery of more of the basic fertilizer values in the sludgev- operation is relatively easyvi- lower capital costvii- suitability for digesting nutrient-rich biosolids

Disadvantages of aerobic digestion: High power cost is associated with supplying the required

oxygen Digested biosolids produced have poorer mechanical

dewatering characteristics The process is affected significantly by temperature, location,

tank geometry, concentration of feed solids, type of mixing/aeration device, and type of tank material

Useful byproduct such as methane is not recovered.

Eqs 14-16 – 14-18:- conversion of organic nitrogen to nitrate results in an

increase in the concentration of H2 ions decrease pH, if sufficient buffering capacity is not available.

- About 7 kg of alkalinity (as CaCO3) are destroyed per each kg of ammonia oxidized. about 50 % of the alkalinity consumed by nitrification can be recovered by denitrification.

Eq 14-19 Eq. 14-20

a) Batch operation type with air additionb) Continuous operation type with air addition

Design criteria for aerobic digester

Figure 14-31Volatile solids reduction in an aerobic digester as a function of digester liquid temperature and digester sludge age (or SRT)

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